1. Field of the Invention
This invention relates to a thick-film gas sensor of a laminar structure and a method of producing the same. More particularly, the invention relates to an improvement in reliability and durability of a thick-film gas sensor of a laminar structure by preventing deterioration of its performance. The sensor comprises a ceramic substrate, a pair of electrodes disposed on the ceramic substrate and a thick porous gas-sensitive film extending across the electrodes while covering the electrode surfaces.
2. Related Art Statement
A typical thick-film gas sensor of a laminar structure of the prior art has a thick gas-sensitive film disposed on a flat substrate, as described in laid-open Japanese Patent Application No. 115,837/85, for example. The gas-sensitive film tends to be peeled off easily due to difference in thermal expansion between the substrate and gas-sensitive film.
The inventors disclosed an improvement in the adhesion of the gas-sensitive film to the substrate in their Japanese Patent Application No. 203,222/83, which improvement was achieved by roughening the substrate surface or by providing artificial undulations on the substrate surface.
Further study by the inventors has revealed that, although the adhesion can be improved on a microscopic scale by the undulation of the substrate surface of the above Japanese Patent Application, there is room for further improvement of the bondage between the substrate and the gas-sensitive film on a microscopic scale. Also, the gas-sensitive film adhered in the manner of the above Japanese Patent Application is apt to produce uneven distribution of internal resistance along the boundary between the substrate and the gas-sensitive film. Moreover, impurities in the substrate and the electrodes tend to migrate to the boundary between the electrodes and the gas-sensitive film during operation, and the internal resistance of the boundary tends to increase accordingly. The uneven distribution of the internal resistance and increased internal resistance of the boundary result in deteriorating the performance characteristics of the gas sensor.